Fossil record of graptoloids challenges the theory that immediately after a mass extinction, surviving species develop new physical traits at a rapid pace

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A colony of Monograptus priodon, a neograptine graptolite
species that arose after the Ordovician mass extinction.
Neograptines did not rapidly evolve until about 2 million years
after the extinction event.

BUFFALO, N.Y. -- Following one of Earth's five greatest mass
extinctions, tiny marine organisms called graptoloids did not begin
to rapidly develop new physical traits until about 2 million years
after competing species became extinct.

This discovery, based on new research, challenges the widely
held assumption that a period of explosive evolution quickly
follows for survivors of mass extinctions.

In the absence of competition, the common theory goes, surviving
species hurry to adapt, evolving new physical attributes to take
advantage of newly opened niches in the ecosystem. But that's not
what researchers found in graptoloid populations that survived a
mass extinction about 445 million years ago.

"What we found is more consistent with a different theory, which
says you might expect an evolutionary lag as the ecosystem reforms
itself and new interspecies relationships form," said University at
Buffalo geology professor Charles E. Mitchell, who led the
research.

The research provides insight on how a new mass extinction,
possibly one resulting from man-made problems such as deforestation
and climate change, might affect life on Earth today.

"How would it affect today's plankton? How would it affect
groups of organisms in general?" asked the paper's lead author,
David W. Bapst, a PhD candidate at the University of Chicago who
studied with Mitchell as an undergraduate. "The general motivation
behind this work is understanding how extinction and evolution of
form relate to each other, and the fossil record is the only place
where we can do these sorts of experiments across long spans of
time."

The research on graptoloids is scheduled to appear online the
week of Feb. 13 in the Proceedings of the National Academy of
Sciences.

Other team members included Peter C. Bullock and Michael J.
Melchin of St. Francis Xavier University in Nova Scotia, and H.
David Sheets of Canisius College in Buffalo. The National Science
Foundation and Natural Sciences and Engineering Research Council of
Canada supported the study.

Graptoloids are an extinct zooplankton that lived in colonies.
Because the animals evolved quickly and had a wide geographic
range, their fossil record is rich -- a trove of information on how
species diversify.

Bapst, Mitchell and their colleagues examined two different
groups of graptoloids in their study: Neograptines and
diplograptines. Each kind lived during the Ordovician mass
extinction that began about 445 million years ago, but only
neograptines survived.

Before the extinction event, diplograptine species were
dominant, outnumbering neograptine species. Diplograptines also
varied more in their morphology, building colonies of many
different shapes.

With diplograptines gone after the Ordovician mass extinction,
neograptines had a chance to recover in an environment free of
competitors.

According to the ecological release hypothesis, a popular
theory, these circumstances should have led to a burst of adaptive
radiation. In other words, without competition, the neograptines
should have diversified rapidly, developing new physical traits --
new colonial architectures -- to take advantage of ecological
niches that the diplograptines once filled.

But that's not what the researchers found.

To test the adaptive radiation idea, they analyzed the colony
forms of 183 neograptine and diplograptine species that lived
before, during or after the Ordovician mass extinction -- a total
of 9 million years of graptoloid history.

This wealth of data enabled the team to track graptoloid
evolution with more precision than past studies could. What the
researchers discovered looked nothing like adaptive radiation.

Almost immediately following the Ordovician mass extinction, new
neograptine species proliferated, as expected. But according to the
study, these new species displayed only small changes in form or
morphology -- not the burst of innovation the release hypothesis
predicts. In fact, graptoloids had been evolving new physical
traits at a more intensive pace prior to the extinction event.

Limited morphological innovation among neograptines continued
for approximately 2 million years after the extinction, Bapst
said.

The lag supports a model of evolution that argues that
interactions between co-evolving species help foster
diversification. Because such relationships likely take time to
develop in a recovering ecosystem, an evolutionary lag of the kind
the graptoloid study detected should occur in the wake of a mass
extinction.

Another possible explanation is that newly appeared graptoloid
species may have differed in ways outside of physical traits, a
phenomenon that biologists refer to as non-adaptive radiations. A
third possibility is that graptoloids may have experienced
evolutionary lag due to their complex mode of growth.

Besides investigating how neograptines fared after the
extinction event, the team also analyzed whether colony form on its
own could explain why neograptines survived the mass extinction
while diplograptines disappeared. The scientists concluded that
this was unlikely, suggesting a role for other factors such as
possible differences in the preferred habitat of the two
groups.